The diagnosis and care of patients with cardiovascular disease critically depends on information about the pumping ability of the heart. For example, the priming pressure of the left ventricle of the heart, typically obtained by measurement of left atrial pressure, indicates, when abnormal, a mismatch between volume capacity of the vascular system and the circulatory blood volume.
Since the early 1970's, the flow-directed pulmonary artery balloon catheter (a.k.a. the Swan-Ganz catheter) has been the standard for bedside hemodynamic monitoring. It yields cardiac output by thermodilution as well as an estimate of mean left atrial pressure. However, under certain conditions, the pressure readings may not faithfully reflect left atrial pressure (R. RAPER et al, "Misled by the Wedge" Chest, March 1986, pp. 427-434). This invasive technique is personnel intensive and costly since the catheter must be inserted and used in a critical care area or operating room, and it has been associated with infection, arrhythmias, and death (E. ROBIN et al, "The Cult of the Swan-Ganz Catheter", Annals of Internal Medicine, Sept. 1985, vol. 103, pp. 445-449). Its use is further limited since it only provides non-automated intermittent measurements, and the catheter should, for safety reasons, only be left in the patient for a few days.
My U.S. Pat. Nos. 5,048,532; 5,181,517; 5,263,485; 5,398,692; 5,551,439; 5,570,671; and 5,697,375, all of which patents are incorporated herein by reference, disclose noninvasive methods and apparatus which includes a catheter containing an inflatable balloon insertable into the esophagus for placement adjacent the left atrium, and associated equipment for making determinations of mean left atrial and mean left atrial transmural pressures. A second catheter containing a second balloon may be used therewith for determining esophageal pressure, which is then added to the mean left atrial transmural pressure to obtain a determination of mean left atrial pressure. Alternatively, a single balloon is used for both purposes wherein the single balloon is moved up the esophagus to measure esophageal pressure and then moved back to the left atrial position. The catheter may also include an esophageal stethoscope and/or an esophageal temperature sensor contained within a protective pouch which surrounds the catheter.
In accordance with one method of positioning the balloon as discussed in my aforesaid U.S. Pat. No. 5,570,671, an electrode means is attached to the catheter just above the balloon to obtain a cardiogram at each of a series of incremental depths as the electrode is moved lengthwise within the esophagus, the esophageal depth at which the balloon is positioned being that which corresponds to the incremental electrode depth at which the electrogram therefor shows the greatest negative portion length of the largest absolute value segment of the respective "p" wave.
However, suitably monitoring a patient's heart condition requires, of course, more than determining the mean left atrial and mean left atrial transmural pressures. Impedance cardiography, which depends on attaching a number of electrodes to the chest, yields continuous readouts of cardiac output, the blood flow generated by the heart. Many researchers consider that this method is unreliable for absolute values but good for relative changes. This device can also provide systolic time intervals. The systolic time interval includes the duration of two phases of ventricular activity: the pre-ejection period (PEP) and the left ventricular ejection time (LVET); PEP refers to the time spent by the ventricle increasing pressure on the volume of blood in it before ejection of the blood into the aorta, and LVET is the duration of the ejection phase. These time intervals are used in various combinations to gauge ventricular performance. For instance, a long PEP is seen when the heart is pumping against increased resistance. Also, the ratio PEP/LVET is known to decrease as cardiac output increases. However, this impedance cardiography method is unable to measure pressures or assess valvular function and furthermore does not work well on the critically ill.
Electrocardiographic analysis of the heart's beating frequency and rhythm allows conclusions about the efficiency of the pump (heart). For instance, an abnormally high beating frequency will preclude effective priming of the pump. In addition, the chambers of the heart must act in synchrony for efficient pumping and, for instance, atrial fibrillation will degrade cardiac output.
Transthoracic and transesophageal echocardiography are excellent techniques for evaluation of valvular function of the heart with respect to leakage and resistance to flow and ventricular pumping action and systolic time intervals. However, the equipment is expensive, the method is very personnel intensive, and the esophageal probe, being large in diameter (perhaps about 9 or 10 mm) is uncomfortable and requires sedation of the patient. This technique also does not lend itself to continuous unattended monitoring of the patient.
Esophageal ultrasound Doppler flow-probe technique provides a good analysis of flow velocity in the descending aorta but gives only partial information about cardiac function. This technique also requires a trained operator. Although it can provide continuous monitoring, small position changes in the probe may make readings unreliable.
Phonocardiography is carried out by placing one or more microphones on the patient's chest. It can give a good analysis of valvular function, but it is susceptible to interference by extraneous sound sources.
Carotid plethysmography offers a qualitative representation of the carotid artery pulse and is used in combination with electrocardiography and phonocardiography to produce systolic time intervals. The method is, however, personnel intensive and not useful for continuous monitoring.
U.S. Pat. Nos. 4,094,308 and 5,086,776 suggest noninvasive methods for sensing cardiac performance. However, these methods are not sufficiently reliable and/or are personnel intensive and/or do not suitably allow continuous automated monitoring of the patient.
The combination of a sphygmomanometer (the common blood pressure cuff), a Swan-Ganz catheter, a phonocardiograph, a carotid plethysmograph, and an electrocardiograph has provided a comprehensive evaluation of cardiac contractility. See "An Indirect Method of Evaluation of Left Ventricular Function in Acute Myocardial Infarction" by C. Agress et al, Circulation, vol. XLVI, Aug. 1972, pp. 291-297. The maximum rate of left ventricular pressure change as a function of left ventricular end diastolic pressure (which equals mean left atrial pressure) has been used to determine cardiac pumping function in patients with myocardial infarction and acute coronary insufficiency with marked predictive value for survivors and non-survivors. Unfortunately, this invasive method suffers from being complicated and cumbersome and carries the risks inherent in cardiac catheterization.